A nitride semiconductor is a wide-gap semiconductor having a wide band gap and is used as a substrate of a laser diode (LD), a semiconductor light emitting diode (LED), or the like by using the wide band gap. The nitride semiconductor has firm crystal and withstands large current and high voltage. Consequently, application to a heterojunction bipolar transistor of high output and the like is also considered.
Challenges of a semiconductor device using such a nitride semiconductor as a substrate are decrease in operation voltage and improvement in reliability. To solve the challenges, realization of a contact electrode structure having low resistance to a nitride semiconductor and high reliability is required.